Properties of small particles, such as size, density and fluorescence, are critical for many applications, but the small size makes measurement difficult. A potentially lethal anthrax spore, for example, has a diameter of one thousandth of a millimeter, a mass of one trillionth of a gram, and is completely invisible to the naked eye. Nonetheless, if anthrax spores were to suddenly appear in a subway it is clearly important to rapidly and accurately detect them. This is a significant challenge. Different applications have different requirements, but the more information an instrument can provide about the particles, the better. Similarly, the speed and accuracy of virtually any such instrument should be maximized while the size, cost and complexity are kept to a minimum. It would thus be desirable to provide a system and method having the ability to measure multiple particle properties more accurately and rapidly than known commercial instruments yet having the potential to be made smaller and more inexpensively.
Particle detection and analysis continue to evolve. One approach uses particle velocity. When a gas travels though a properly designed pressure gradient, entrained particles are focused and accelerated to a terminal velocity dependent upon their aerodynamic diameter. Measurement of the subsequent particle velocity is thus a means to obtain accurate aerodynamic particle size data. This basic technique has been employed for some time in a range of instruments. The velocity is generally determined by measuring the time it takes for a particle to pass between two laser beams separated by a known distance. Because of noise and finite clock speed, velocity is determined most accurately when the spacing (and hence transit time) between laser beams is large. As the mean spacing between particles becomes comparable to the spacing between lasers, however, the system becomes increasingly confused by coincident particles, crossing particles and related phenomena. Unfortunately, high particle loads must be dealt with in many applications. To handle such loads the lasers can be moved closer together (reducing sizing accuracy) or more lasers can be used (increasing cost). The Aerodynamic Particle Sizer (APS) from TSI is an example of an instrument in which two lasers are put close together to enable high analysis rates with rather low accuracy. The Aerosol Time of Flight Mass Spectrometer (ATOFMS) from TSI is a system in which two lasers are relatively far apart, which enables accurate sizing but a low analysis rate. The BioAerosol Mass Spectrometry (BAMS) system developed at LLNL uses six lasers to achieve both high accuracy and a high analysis rate, but the system is complex and expensive.
Therefore, it would be desirable to determine aerodynamic size and trajectory with high accuracy and speed but with minimal complexity and cost.
The aerodynamic size of a particle is related both to its physical size and to its density. The relative importance of the physical and aerodynamic sizes vary from application to application, but measuring both of them simultaneously would be useful in that it provides information on the particle's mass density, which may be very useful. As with the aerodynamic size, many techniques and instruments have been developed to measure physical size. Optical techniques include measuring the total amount of light scattered from a particle, the spatial pattern of scattered light and other scattered light properties, but very few techniques measure both sizes simultaneously. At least a few instruments, such as the APS, ATOFMS and BAMS, are in theory capable of making such measurements, but generally in these instruments the physical size measurement is rather inaccurate being based upon the total amount of light scattered from a particle in a laser beam.
Therefore, it would be desirable to determine the aerodynamic size, physical size and consequently mass density with higher accuracy and speed but with minimal complexity and cost.
Single particle fluorescence has been measured in several ways. The Hach Homeland Security Technologies BioLert system measures single particle fluorescence with a single continuous wave (CW) laser and minimal collection optics. The fluorescence signal produced and detected is very weak and accurate quantification is a challenge. The system is, however, relatively inexpensive. General Dynamic's Biological Agent Warning System (BAWS) uses a randomly-fired, pulsed laser to deposit more energy into a particle, which does produce more fluorescence, but cost is increased. The BAMS system uses an even more powerful pulsed laser triggered by a separate tracking system and carefully designed optics to collect as much fluorescence as possible, but the result is a very expensive system. Other fluorescence systems exist as well.
Therefore, where desired, it would be desirable to determine a particle's fluorescence with high accuracy and speed but with minimal complexity and cost.